Microelectromechanical systems (“MEMS”) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, many such MEMS devices often have a structure suspended above a substrate, and associated circuitry that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
When a voltage is applied to a MEMS device, a surface charge may build up within the MEMS device (e.g., within a surface of the capacitor of the MEMS). This surface charge can impact the performance and accuracy of the MEMS device by redistributing the electric field. This redistribution of the electric field can cause the performance of the device to drift, and can severely restrict the range of stable operation of some MEMS devices. In more severe instances, the trapped surface charges can cause stiction and device failure.
Prior attempts to measure the surface charge have utilized Klein probe force microscopy (KPFM). However, the KPFM technique is destructive (e.g., the MEMS device is destroyed during testing), and has a very low scan rate. Other prior art methods measure the charge/discharge current of a battery, but are not sensitive enough to measure the surface change within a MEMS device.